The kinetics of primary nanocrystallization of an Al 85 Ni 5 Y 8 Co 2 metallic glass, with quenched-in nuclei and a distinct glass transition temperature (T g ) upon heating, was studied by X-ray diffraction, differential scanning calorimetry, in situ electrical resistance measurement and high-resolution transmission electron microscopy. It was identified that a glass transition occurs before onset of primary crystallization reaction although quenched-in nuclei do exist in this glass. The formation and the evolution of the primary Al nanocrystals proceed at a separated mode: the growth of a few quenched-in nuclei of Al occurs predominantly well below T g , followed by a high density nucleation in the residual amorphous matrix around T g and finally growth of all nuclei developed before. This result provides a new insight into the microstructural control in Al-based metallic glasses.